This pilot study will examine the use of Citrulline as a marker of rejection in Intestinal Transplantation. Intestinal Transplants are being performed when the gut is not able to support the nutritional needs of a patient. They are increasingly more frequent and more successful. Medicare recently funded their performance in selected Programs. Rejection remains the most common complication of Intestinal Transplantation. To date, the timely diagnosis of rejection, which is essential for successful treatment, can only be made with endoscopic examination and biopsy. Preliminary data obtained by the investigators suggest that Citrulline, an amino acid that is synthesized almost exclusively by the intestine, is a marker of rejection. It seems that the serum levels of Citrulline drop proportionally to the degree of the rejection. The proposed study will correlate the Citrulline measurements with those of contemporaneous endoscopic and histological examinations. If the findings are favorable it will be expanded to multiple centers. Discovery of a serum marker of intestinal rejection will make its detection possible without invasive tests, cheaper even from a distance. It will improve the safety and outcomes of intestinal transplants.